1. Field of the Invention
The present invention relates to methods of and systems for reducing NOx emissions from the exhaust of an internal combustion engine and, in particular, to methods of reducing NOx emission by adjusting the NO to NO2 ratio in the exhaust before the exhaust gases flow through an SCR catalyst.
2. Background Art
Emissions of NOX and particulate matter (PM) are of primary concern for both diesel and gasoline vehicles to meet future emissions standards. Diesel vehicles have significant advantages over their gasoline counterparts including a more efficient engine, higher fuel economy, and lower emissions of HC, CO, and CO2. For example, diesel vehicles potentially have a 40% higher fuel economy than current gasoline vehicles with 20% lower CO2 emissions.
Control of NOx onboard a diesel vehicle is not a trivial task due to the high oxygen content of the exhaust gas. Such high oxygen fuel systems are typically referred to as lean burn systems. In such lean burn systems, NOx control is more difficult because of the high O2 concentration in the exhaust, making conventional three-way catalysts ineffective. The available technologies for NOx reduction in lean environments include Selective Catalytic Reduction (SCR), in which NOx is continuously removed through active injection of a reductant over a catalyst and Lean NOx Traps (LNT), which are materials that adsorb NOx under lean conditions and must be periodically regenerated by running under rich conditions. Technologies utilizing an ammonia-based reductant, such as aqueous urea, have shown potential in achieving high NOx conversion with minimal fuel economy penalty. Selective Catalytic Reduction (SCR) with ammonia as the reductant has been used extensively for stationary source NOx control. The high selectivity of ammonia for reaction with NOx in high O2 environments makes SCR attractive for use on diesel vehicles. Compared to ammonia, aqueous urea is much easier for use onboard a vehicle. Although such SCR catalysts show great potential for NOx control, utilization of such catalysts are adversely affected by the presence of hydrocarbons in a vehicle exhaust. Specifically, hydrocarbons are known to poison most SCR catalysts.
Various attempts have been made to understand and improve the performance of SCR catalysts. For example Koebel et al. discuss the reactions involved in the reaction between ammonia and NOx. (Koebel et al., SAE Technical Paper Series, 2001-01-3625, 2001). Koebel shows that the main reaction between ammonia and NO is:4 NH3+4 NO+O2—>4 N2+6H2O 
For this reaction, 1 mole of NO will consume 1 mole of ammonia and ¼ mole of oxygen. Koebel goes on to explain that a faster reaction is described by:4 NH3+2 NO2—>4 N2+6H2O 
Accordingly, it is known that the performance of a NOx conversion system can be improved by increasing the fraction of NO2 in the exhaust up to an optimal amount so that the fraction of NO2 does not exceed 50%. Koebel states that this can be accomplished by oxidizing the NO with a strong oxidation catalyst such as platinum. Although urea is the preferred reducing agent for SCR catalysts, ammonia is still likely to be the ultimate reductant since urea rapidly liberates ammonia in the SCR catalyst.
Although the increased NO2 is desirable in a vehicle exhaust, the prior art fails to teach a method for systematically and reliably attaining an optimal NO to NO2 ratio in the exhaust. Furthermore, the prior art fails to teach a method that can be adjusted to give optimal performance for a given type of automobile engine.